NASA has just a little bit more information. Some bright material that was photographed four days ago has disappeared, leading scientists to believe it was ice that vaporized when exposed.

"It must be ice," said Phoenix Principal Investigator Peter Smith of the University of Arizona, Tucson. "These little clumps completely disappearing over the course of a few days, that is perfect evidence that it's ice. There had been some question whether the bright material was salt. Salt can't do that."

The chunks were left at the bottom of a trench informally called "Dodo-Goldilocks" when Phoenix's Robotic Arm enlarged that trench on June 15, during the 20th Martian day, or sol, since landing. Several were gone when Phoenix looked at the trench early today, on Sol 24.

There will be a press conference Friday at 1PM EDT with more details. Link

Update: Wired has the two photographs in gif form so you can see the difference. http://blog.wired.com/wiredscience/2008/06/mars-phoenix-tw.html